Recommended for
Roof Area between 1000 to 1500 sq. meters.
Annual rainfall up to 800 mm. with a daily limit up to 30 mm.
Existing Single borewell with yield
> 2820 lits/hour indicating presence of productive shallow/ deep aquifer/s.
Methodology
Channelize the rainwater collected on the roof of the building/s into one or two vertical drainpipes of at least 4” in diameter. Bring this water through a single horizontal drainpipe of at least 4” diameter, laid down over or below the land surface. Join this drainpipe to the inlet pipe. If required, construct a small chamber of 0.5 m x 0.5 m x 0.5 m dimension for installation of a two-way valve.
Install a two-way ball bearing valve to the drainpipe, where it connects the inlet pipe of the filter media. Close this valve during the first spell of rain and allow the rooftop water to go into the drain chamber. Also close this valve during the high intensity rainfall spells.
Using RCC slab, construct a tank of 8 m x 2 m x 3 m (L x W x D) size with two chambers, one for settling & the other for filtration of rooftop rainwater. In continuation with this, construct a third chamber (inspection) of 2 m x 2 m x 2.5 m size to accommodate the existing borewell. Hence, take due precautions so that the borewell is not damaged during the construction of the chamber. Please refer to the drawing for further detail.
This tank is to be constructed below the ground. Hence, it is necessary to cover the tank with an RCC slab. Provide manholes within the slab as shown in the drawing.
Construct a partition wall between the settling and filtration chambers. In the filtration chamber create a filter bed composed of Sand, Gravel, Activated Charcoal, & Pebbles. This filter material is readily available in the markets.
Initially lay a 600 mm layer of pebbles at the bottom of the filter. Cover it with silicon mesh of < 70 micron from the top. Over this silicon mesh place a 750 mm thick uniform size layer of gravel (size 3 to 5 mm). This is also separated by a silicon mesh of the same size. Add an activated charcoal layer of 150 mm thickness over this followed by silicon mesh again. Put a sand layer of 600 mm thickness on the top, wrapped in silicon mesh. For more details, refer to the drawings.
For carrying the filtered & clean water from the filtration chamber, lay down a 2” dia PVC pipe at the bottom of the inspection chamber. Join this pipe with the borewell casing pipe by making an appropriate size hole into it. Put a two-way valve in the middle. This valve is also to be connected to the drain chamber. This valve is to be opened when the borewell starts overflowing.
In the settling chamber provide two overflow pipes to drain out the excess water collected in it. This is required for high intensity rainfall spells. This helps to maintain the appropriate hydrostatic pressure within the settling chamber and passes the water into the filtration chamber.
Process
The rooftop channelised water will come through the inlet pipe into the settling chamber. This water is allowed to fall over a small slab (8” long and 2” thick) to avoid the erosion of the tank walls. This slab will also help in reducing the velocity of the inflow water. Slowly the settling tank will get filled by the roof top rainwater. Soon after the water level in the chamber reaches the top of the partition wall, water will start flowing into the filtration chamber.
As soon as the water column increases into the settling chamber, slowly the water becomes stagnant. This helps in settling the silt load present in the settling chamber itself.
When the water enters the filtration chamber, it will start spreading over the sand layer and because of high porosity it will go down naturally. During this movement the heavier clay and silt particles present in the water will start depositing into the pore spaces present in the sand and gravel layers. After entering the pebble layer, the water will settle down and remaining dust, clay and silt particles will get deposited at the bottom of the filter. Only clean water will come up.
The activated charcoal helps in adsorption of the chemical and bacterial contaminants present in the water. Hence, it is the most important and non-negotiable constituent of the artificial filter bed.
After saturation of the pebble layer, the clean water will slowly start flowing through the outlet pipe and enter into the existing borewell. The velocity of this water will be very less. Hence the aquifer/s present within the borewell will accept continuously.
Precautions
During the first rainfall spell, close the inlet pipe two-way valve and drain the runoff water. This will help in avoiding the heavy contaminant load entering the chambers. During dry spells (dry days > 10) within the monsoon adopt the same procedure.
During high intensity rainfall spells (>25 mm/hour) open the inlet pipe two-way valve, so that maximum roof top water will drain directly into the drain chamber.
When inflow of water increases in the settling chamber and outflow from the filtration chamber decreases, excess water collected there will automatically get drained out through two overflow pipes given in the settling chamber.
If the borewell starts overflowing, then immediately close the outflow valve and drain the excess water into the drain chamber. At the time of cleaning the filter keep the outflow valve closed. This will protect the borewell from siltation, contamination, or damage.
Carry out the flushing of existing borewell in alternate years.
For low yielding (between 1375 to 2020 lph) existing borewells reduce the inflow water by 50%. i.e. reduce the catchment area of the roof and accordingly use only one vertical inflow pipe for rooftop water.
Maintenance
Every year, it is necessary to clean the roof of the building, settling chamber, and filter chamber prior to the monsoon.
Immediately after the monsoon is over, take out the stagnant water in the settling chamber. Also take out the silt settled at the bottom of the tank. For cleaning, use the manhole placed between the settling and filtration chambers.
Every year during March-April take out the upper layer of sand and wash it thoroughly. After due drying, top up the sand layer with this washed sand.
Once in two years take out the complete filter bed i.e. sand, activated charcoal, gravel and pebble layers. They are to be cleaned, washed, dried properly, and then place in a filtration chamber. Replace the activated charcoal after 3 to 4 years.
The society/management of the building should train their personnel for the annual maintenance of the filter and make appropriate provision of funds.
Every year check the water quality parameters (chemical and bacteriological) of the borewell water at least during pre- and post-monsoon seasons.